REFRIGERATION DEVICE

Abstract

 Refrigeration device, comprising:
- at least one laser light device (1) for generating laser light,
wherein there are provided
- at least one hollow-core waveguide (2)
- at least one optical coupling device (3) for coupling laser light generated by the at least one laser light device (1) into the at least one hollow-core waveguide (2)
- at least one buffer gas source (4) which is or can be connected to the at least one hollow-core waveguide (2) for providing buffer gas into the hollow-core of the hollow-core waveguide (2) and at least one cooling gas source (5) for cooling gas which is or can be connected to the at least one hollow-core waveguide (2) for providing cooling gas into the hollow-core of the hollow-core waveguide (2) such that during operation of the refrigeration device atoms or molecules of the buffer gas and atoms or molecules of the cooling gas are present together and collide with each other in the hollow-core of the at least one hollow-core waveguide (2).

Description

[0001] The present invention relates to a refrigeration device for the refrigeration of a gas mixture of a cooling gas and a buffer gas and its surrounding material by using laser light having the features of the preamble of claim 1, a refrigerator comprising at least one such refrigeration device and a method for cooling gas using laser light having the features of the preamble of claim 11.

[0002] Lasers are sources of energy which is highly concentrated in space and momentum at almost zero entropy. This effective very low temperature is very successfully used to cool dilute atomic gases to almost arbitrarily close to absolute zero. In addition, laser light generated optical traps can be considered to have zero temperature walls. Similarly, opto-mechanical cooling of single eigenmodes of microscopic mechanical oscillators can reach the quantum ground state. Despite long standing efforts, the applications towards cooling molecular gases, liquids or in particular whole solid objects, however, proved significantly more difficult to implement. Already two decades ago the first basic proof-of-concept implementations used selective doping to cool a whole optical fiber by tens of degrees. Anti-Stokes Raman light scattering of the guided field modes was used to strongly depopulate phononic or motional modes. Unfortunately, non-radiative emission processes and reabsorption of the anti-Stokes photons by pollutions in the material as well as heating from the environment and support structures prevented to reach much lower temperatures. In the following generations of experiments, the use of isotope purified small crystals and improved environmental shielding finally allowed to reach cryogenic temperatures of macroscopic objects, significantly beating the temperature limits achieved via thermo-electric cooling.

[0003] A solid-state refrigeration device according to the prior art is discussed in US 8,720,219 B1.

[0004] Despite these significant improvements over the last two decades, the efficiency of the cooling process remained rather low, reaching only a couple of Milliwatt (mW) of cooling power from 50 Watt (W) of laser power. As one central reason for this low efficiency one can identify the rather limited Stokes shift of the emitted phonons of less than 1/1000 of their frequency. Hence one needs thousands of successful cycles with nonradiative decay per background absorption.

[0005] Interestingly, it has been shown recently that this ratio can be tremendously improved using excited molecular states (exciplexes) of Alkali-rare gas molecules as e. g. Rb-Ar or K-Ar which do not have bound ground states (U. V. A. Saß and M. Weitz, "Laser cooling of a potassium-argon gas mixture using collisional redistribution of radiation." Appl Phys B 102, 503-507 (2011)). The Alkali atoms play the role of the cooling gas while the Ar atoms are the buffer gas at high pressure to insure the formation of exciplexes. The experiments are, however, limited to geometries unfavorable to macroscopic refrigeration (gas cell) with specific mixtures (Rb-Ar or K-Ar) where cooling is observed as spectral narrowing of the emission lines of Rb or K atoms. It is therefore desirable to identify optimal cooling geometries and optimal exciplex gas components with the aim of providing a refrigeration mechanism for the larger environment and not only of local alkali environments within the exciplex gas.

[0006] It is an object of the invention to provide for a refrigeration device and a method for cooling a cooling gas in which efficiency of the cooling process is increased.

[0007] This object is being achieved by a refrigeration device with the features of claim 1, a refrigerator comprising at least one such refrigeration device and a method with the features of claim 11.

[0008] The invention is based on the particular property of exciplexes formed, e. g., by alkali atoms colliding with noble gas atoms, which do not bind in the ground state but instead exhibit a transitory bound excited state. During the small time-window of exciplex formation, photons generated by the at least one laser light device (of energy ℏω_L) and coupled into that at least one hollow-core waveguide by way of the at least one optical coupling device (e. g. in the form of an optical lens or lens system), which are energetically matched to the instantaneous binding energy can be absorbed. During subsequent dynamics, the exciplex becomes unbound and upconverted blue-shifted photons at an energy ℏω₀, which correspond to the bare transition of the cooling gas and which is larger than ℏω_L, can be spontaneously emitted. An overall energy loss of ℏΩ = ℏω₀ - ℏω_L > 0 per cycle occurs in the gas mixture, leading to a reduction of temperature of the cooling gas and its surrounding material. The process efficiency is described by the parameter

which can reach much larger values (∼ 10 %) compared to similar cooling schemes.

[0009] The use of hollow-core waveguides provides that laser light is very efficiently used for the cooling process as the light travels mostly in the gas filled core over a long interaction length. The length of the at least one hollow-core waveguide(s) can be chosen such that an input power I₀ of laser light coupled into the at least one hollow-core waveguide is being reduced to a much lower power I_out by light absorption in exciplexes and scattering out of the hollow-core waveguide (ideally for long waveguides I_out will be close to zero). Two important advantages are offered by the one-dimensional geometry of the hollow-core waveguide:

• unwanted light absorption by material of the waveguide is very low as light does not propagate through the boundary material
• spontaneously emitted photons are upshifted in energy to frequencies which are not supported by the waveguide thus effectively cancelling unwanted reabsorption in the gas mixture

[0010] It is preferably provided that the density of buffer gas inside the hollow-core waveguide is chosen to be much larger (e. g. by at least a factor of 10, 100 or 1000) than the density of the cooling gas thereby reducing the number of collisions between the atoms or molecules of the cooling gas.

[0011] It is of course not necessary to provide separate sources of cooling gas and buffer gas wherein each of the sources is separately connected to the at least one hollow-core waveguide(s). There could also be a joint or common gas source of cooling gas and buffer gas containing these gases in pre-mixed form. In such an embodiment both, the cooling gas source and the buffer gas source are being embodied by the joint or common gas source.

[0012] The buffer gas should be chosen to be chemically inert with respect to the cooling gas. Suitable buffer gases could be noble gases.

[0013] The cooling gas and/or the buffer gas could be composed of single chemical elements or could be composed of chemical compounds, molecules or the like.

[0014] The hollow-core waveguide(s) can be arranged such that it/they run along a straight line or it/they could be bent. It is also possible to use hollow-core waveguide(s) which is/are bent into a circle such that the hollow-core(s) provide(s) an endless space. The optical coupling device can be arranged at one specific position on the circumference of the waveguide(s). It is also possible to use more than one optical coupling device.

[0015] In an embodiment of the invention the at least one buffer gas source is configured to provide a noble gas, preferably argon.

[0016] In an embodiment of the invention the at least one cooling gas source is configured to provide alkali gas, preferably Rubidium, or halogen gas, preferably chlorine gas or fluoride gas.

[0017] In an embodiment of the invention the at least one hollow-core waveguide is made of dielectric material and/or is in the form of a fiber, preferably in the form of a photonic crystal fiber.

[0018] In an embodiment of the invention there is provided a plurality of hollow-core waveguides which are bundled together (and possibly to each other) and which preferably are coupled to the same laser light device(s). This improves cooling by increasing the volume to surface ratio of the refrigeration device.

[0019] In an embodiment of the invention a pressure of a mixture of the atoms or molecules of the buffer gas and the atoms or molecules of the cooling gas in the at least one hollow-core waveguide is in a range of about 1 bar to about 50 bar, preferably about 5 bar to about 25 bar.

[0020] In an embodiment of the invention a length of the at least one hollow-core waveguide is chosen such that at least 10 %, preferably at least 50 %, of the power of the laser light coupled into the at least one hollow-core waveguide is being absorbed via exciplex-formation present in the at least one hollow-core waveguide.

[0021] In an embodiment of the invention a diameter of the hollow-core of the at least one hollow-core waveguide is in a range of about 5 µm to about 100 µm, preferably in a range of about 15 µm to about 25 µm.

[0022] In an embodiment of the invention the at least one laser light device for generating laser light is configured to generate laser light with a wavelength in a range of about 200 nm to about 1000 nm, preferably with a wavelength in a range of about 500 nm to about 700 nm.

[0023] In all of the above-referenced embodiments it is possible that the refrigeration device is used to cool the cooling gas as a primary object of its operation. Of course, it is also possible to use the fact that the cooling gas cools the walls of the at least one hollow-core waveguide to operate a refrigerator comprising at least one refrigeration device according to at least one of the discussed embodiments.

[0024] The at least one hollow-core waveguide provides a quasi-one-dimensional geometry for the formation and dynamics of the exciplexes.

[0025] A surprising result is that increasing buffer gas density (as a means to increase the frequency of collisions) is not always an optimal strategy as it can reduce cooling rates.

[0026] In an embodiment of the invention it is provided that the hollow-core of the waveguide is closed on both ends of the waveguide with respect to the cooling gas and the buffer gas (but is of course open to receive the laser light generated by the at least one laser light device and coupled into the hollow-core by the optical coupling device). In this case it can be provided that the hollow-core waveguide has at least one opening which can be opened and closed in order to release the cooled cooling gas.

[0027] In an embodiment of the invention the gas mixture of cooling gas and buffer gas is pumped through the fiber entering at ambient temperature on one end and exiting at much lower temperature on the other end. Temperature limit of the gas mixture can be much lower as for Peltier (< 150 K) and the gas mixture can be used to cool other devices.

[0028] Embodiments of the invention are being described with respect to Fig. 1 - 4:

Fig. 1 shows schematically an embodiment of the invention comprising a multimode hollow-core waveguide 2 in the form of a fiber of inner radius r and length I filled with Rb-Ar gas which is pumped by laser light provided by a laser light device 1 at central frequency ω_L with bandwidth δω and input power I₀. Cooling gas (by way of example Rubidium - Rb) is provided into the hollow-core of the hollow-core waveguide 2 by a cooling gas source 5. Buffer gas (by way of example Argon - Ar) is provided into the hollow-core of the hollow-core waveguide 2 by a buffer gas source 4. Other than shown, the cooling gas and the buffer gas could be pre-mixed outside the hollow-core waveguide 2 such that the gas mixture of cooling gas and buffer gas could be provided to the hollow-core waveguide 2.
The laser light generated by the laser light device 1 is coupled into the hollow-core waveguide 2 by an optical coupling device 3 which is known in the prior art (e. g. one or more optical lenses). Power loss resulting in a reduction of the kinetic energy of the cooling gas occurs via spontaneous emission from the Rb atoms leading to a reduced power I_out of laser light leaving hollow-core waveguide 2.

Fig. 2 shows the dynamics of an inventive cooling process for an example working with a Rb-Ar collision process showing a ground state Ar atom approaching a Rb atom initially in the ground state at constant velocity v. On the y-axis ground state potential U_g(a) and the excited state potential U_e(a) are shown. On the x-axis the separation between the Ar atom and Rb atom is shown. During the collision time τ ≃ 2δa/v, the laser light is resonant to the transition between distances a₀ ± δa where a₀ is the coordinate of the minimum of U_e(a). Following absorption at frequency ℏω_L ≃ U_e(a₀) - U_g(a₀) an exciplex is formed with a lifetime of τ_γ = γ^-1. Spontaneous emission at rate γ leads to an effective energy loss.

Fig. 3 shows a table with parameters for an embodiment of the invention using Rb as cooling gas and Ar as buffer gas. An initial temperature resulted in a relative collision velocity of about v = 433 m/s. Spontaneous emission rate γ = 2π × 5.75 MHz, loss per cycle is about Ω = 2π × 6,7154 THz. The bare ground-excited frequency difference is ω₀ = 2π × 377 THz. The simulations in Fig. 4 have been done for these parameters.

Fig. 4a shows a sketch of a bundle of N hollow-core waveguides 2 in the form of fibers as an arrangement that minimizes heat flow while maximizing cooling power leading to a temperature drop roughly scaling as

where δT is the single fiber temperature drop.

Fig. 4b shows the temperature vs. radius for a single hollow-core waveguide 2.

Fig. 4c shows the temperature plotted in grey values (darker grey means lower temperature) for a bundle similar to the one shown in Fig. 4a over a section orthogonal to the extension of the bundle.

Fig. 4d shows the temperature in Kelvin for y = 0 in Fig. 4c.

List of Reference Signs:

[0029] 

1

laser light device

2

hollow-core waveguide

3

optical coupling device

4

buffer gas source

5

cooling gas source

ω_L

frequency of laser light

δω

bandwidth of laser light

I₀

input power of laser light entering hollow-core waveguide

I_out

power of laser light leaving hollow-core waveguide

ω₀

transition frequency between bare excited and bare ground state of cooling gas

ℏΩ

energy loss of cooling gas per spontaneous emission event

U_g(a)

ground state potential

U_e(a)

excited state potential

a₀

coordinate of the minimum of U_e(a)

δa

variance in distance around a₀

τ

collision time

τ_γ

lifetime of exciplex

γ

spontaneous emission rate

v

velocity

Claims

1. Refrigeration device, comprising:

- at least one laser light device (1) for generating laser light characterized in that there are provided

- at least one hollow-core waveguide (2)

- at least one optical coupling device (3) for coupling laser light generated by the at least one laser light device (1) into the at least one hollow-core waveguide (2)

- at least one buffer gas source (4) which is or can be connected to the at least one hollow-core waveguide (2) for providing buffer gas into the hollow-core of the hollow-core waveguide (2) and at least one cooling gas source (5) for cooling gas which is or can be connected to the at least one hollow-core waveguide (2) for providing cooling gas into the hollow-core of the hollow-core waveguide (2) such that during operation of the refrigeration device atoms or molecules of the buffer gas and atoms or molecules of the cooling gas are present together and collide with each other in the hollow-core of the at least one hollow-core waveguide (2)

and in that the buffer gas provided by the at least one buffer gas source (4) and the cooling gas provided by the at least one cooling gas source (5) are chosen such that in the presence of laser light generated by the at least one laser light device (1) and due to the collisions the atoms or molecules of the cooling gas and the buffer gas form exciplexes, which do not bind in the ground state, but instead exhibit a transitory bound excited state and thermic energy of the atoms or molecules of the cooling gas is being converted to photons spontaneously emitted during the decay of the exciplexes thereby cooling the atoms or molecules of the cooling gas.

2. Device according to claim 1, wherein the at least one buffer gas source (4) is configured to provide a noble gas, preferably argon.

3. Device according to one of the preceding claims, wherein the at least one cooling gas source (5) is configured to provide alkali gas, preferably Rubidium, or a halogen gas, preferably chlorine gas or fluoride gas or an excimer gas mixture

4. Device according to one of the preceding claims, wherein the at least one hollow-core waveguide (2) is made of dielectric material and/or is in the form of a fiber, preferably in the form of a photonic crystal fiber.

5. Device according to one of the preceding claims, wherein there is provided a plurality of hollow-core waveguides (2) which are bundled together and which preferably are coupled to the same laser light device (1).

6. Device according to one of the preceding claims, wherein a pressure of a mixture of the atoms or molecules of the buffer gas and the atoms or molecules of the cooling gas in the at least one hollow-core waveguide (2) is in a range of about 1 bar to about 50 bar, preferably about 5 bar to about 25 bar.

7. Device according to one of the preceding claims, wherein a length of the at least one hollow-core waveguide (2) is chosen such that at least 10 %, preferably at least 50 %, of the power of the laser light coupled into the at least one hollow-core waveguide (2) is being absorbed via exciplex-formation present in the at least one hollow-core waveguide (2).

8. Device according to one of the preceding claims, wherein a diameter of the hollow-core of the at least one hollow-core waveguide (2) is in a range of about 5 µm to about 100 µm, preferably in a range of about 15 µm to about 25 µm.

9. Device according to one of the preceding claims, wherein the at least one laser light device (1) for generating laser light is configured to generate laser light with a wavelength in a range of about 200 nm to about 1000 nm, preferably with a wavelength in a range of about 500 nm to about 700 nm.

10. Refrigerator comprising at least one refrigeration device according to at least one of the preceding claims.

11. Method for cooling gas in which the gas to be cooled is mixed with a buffer gas which is chosen such that such that the atoms or molecules of the cooling gas and the atoms or molecules of the buffer gas do not bind in the ground state but instead exhibit a transitory bound excited state characterized in that the mixture is irradiated with laser light in at least one hollow-core waveguide (2) such that light exciplexes are formed by the laser light and by the collisions of atoms or molecules of the cooling gas and atoms or molecules of the buffer gas and thermic energy of the atoms or molecules of the cooling gas is being converted to photons spontaneously emitted during the decay of the exciplexes thereby cooling the atoms or molecules of the cooling gas.

Drawing